We have continued our analysis of assembly and maturation of HIV-1 and murine leukemia virus (MLV) particles. Specifically, we have characterized the properties of the HIV-1 Gag protein, the fundamental building block of the HIV-1 virus particle. We found that the Gag protein is in monomer-dimer equilibrium in solution. We identified the interface responsible for the dimerization; it is a site previously described as the site at which the HIV-1 capsid protein (which forms a portion of Gag) dimerizes. We also found that inositol hexakisphosphate (IP6), which modulates the nature of the virus-like particles assembled from Gag when nucleic acid is added, causes a shift from monomer-dimer to monomer-trimer equilibrium. Several lines of evidence indicate that when both nucleic acid and IP6 are present, the nucleic acid binds to the C-terminal, nucleocapsid (NC) domain of the protein while the IP6 binds to the N-terminal matrix (MA) domain. By mutating the dimer interface in Gag, we generated a protein which remains monomeric in solution. We then used a wide variety of experimental approaches, including gel filtration, static and quasi-elastic light-scattering, sedimentation velocity analysis, and small-angle neutron scattering, as well as molecular modleing, to characterize the conformation of this protein. All of the data indicate that the protein is folded over in solution, with its ends near each other in 3-dimensional space. In contrast, the protein is known to be a highly extended rod in assembled virus particles; thus, it must undergo a major conformational change when it assembles. We have begun a similar analysis of the MLV Gag protein. Remarkably, preliminary evidence indicates that the properties of this protein are quite different from those of HIV-1 Gag. [Corresponds to Rein Project 1 in the April 2007 site visit report of the HIV Drug Resistance Program]